Tidal power devices offer advantages over wave power devices: tides are regular and predictable, whereas wave power depends essentially upon weather conditions; and lower device masses can be employed at coastal sites, because locations for tidal devices are generally exposed to less extreme weather and the devices do not therefore have to be constructed to the same level of survivability as ocean wave power devices.
Several major tidal power schemes have been constructed in river estuaries, but these have involved barrages and major engineering works, and they risk permanent changes to ecosystems in areas of importance to wildlife. They are extremely costly to build, and it is increasingly difficult to find suitable locations for such schemes.
There have been several proposals for smaller-scale, in stream, tidal power devices that have less environmental impact. For example:
Wo2005078276 discloses a device which floats in a tidal flow beneath the water surface and employs multiple turbines arranged on horizontal axes.
BE894952 discloses a turbine system in a moored floating pontoon. It has three directing channels, two of which are rectangular side ducts and the other an open duct, which direct the water flow on to a vertical axis, planar and hinged bladed water wheel.
GB2153917 discloses a system in which tidal lagoons feed two vertical axis planar bladed rotors in a pontoon floating in a specially-constructed chamber.
GB2337305 relates to a large vertical axis rotor with aerofoil blades to cause rotation of the floating structure relative to a tethered ring cam pumping mechanism.
DE10026674 relates to apparatus having a number of horizontal axis, planar bladed, rotors mounted in front of a weir and floating up and down under compressed air control in line with the water levels.
RU2160848 concerns a floating power station in which a large horizontal axis ‘blind’ rotor similar to a paddle steamer's is carried on a floating bridge or pontoon.
FR2867523 discloses an array of vertical axis turbines carried beneath a tethered barge, but without any ducting.
US2006008351 relates to a device having a row of horizontal axis Darrieus rotors driving a surface generator through a 90 degree gearbox.
FR2369440 discloses a floating pontoon which seeks to exploit both tidal flows and wave movement. The turbines are slung beneath the pontoon, but can also be supplied with water collected by wave movement flowing downwardly from the pontoon and on to the turbines.
GB2312931 discloses a tidal or wave generator having an upright rotor in a channel housing which includes pivoting baffles which move between a closed position directing flow towards the rotor, and an open position against the wall of the channel.
Similarly, WO2005/035977 discloses a tidal generator with pivoting baffles directing flow to one side of the rotor while swinging out of the way when downstream.
WO92/21877 discloses an apparatus in which the rotor is mounted on a horizontal axle across the channel, with reversible baffles mounted on horizontal axles and intended to direct flow to one side of the rotor, while swinging out of the way when downstream.
The present invention seeks to provide a simple and cost-effective apparatus for extraction of tidal energy.